There have been various proposals for using a laser beam as a reference that impinges on a target which is mounted to a structure, or a part of a structure under surveillance. As the structure moves the target moves with it. The beam provides a spatial reference against which displacements of the target are detectable. It is, or course, possible to mount the laser source to whatever is to be monitored. Normally, however, the target will be fixed to the surveyed structure liable to movement and the laser source mounted to a fixed point.
Many of the proposals provide for a laser source to direct its beam at a target which, for example, comprises some form of photocell assembly whereby the position of the beam on the assembly can be detected. Examples of this type can be found in British Patent Specifications Nos. 1,178,007: 1,313,416 (in which refractive displacement of the beam is measured): 1,323,104 (which is concerned specifically with building structures): 1,338,167 (which is also concerned with alignment in building structures): 1,372,145: 1,436,740 (which is for contour measurement as the target is moved along a path): 1,513,380: and in PCT publication No. W081/03698. Except for specification 1,372,145, all the above-mentioned specifications essentially measure a relative laterial displacement between the laser source, which is usually taken to be mounted on a location taken as fixed, and the target which is on the part subject to displacement. Specification No. 1,372,145 (U.S. Pat. No. 3,799,674 to Guillet et al) shows monitoring of a dam in which apparent deflections of the beam relative to two targets mounted on the dam wall are detected in respective orthogonal directions by respective targets. It will be appreciated that in general the optical system should be kept as simple as possible to avoid unwanted displacements due to relative movement of parts of the system, such as arising from temperature changes.
There has arisen a need to be able to monitor structures at various points for signs of movement with apparatus that requires little attention and supervision. One system for monitoring an elongate structure at various points along its length is disclosed in British patent specification No. 2,101,305 in which displacements in the hull of a tanker are monitored by directing a laser beam along the tanker and locating a reflecting target at points along its length. Measurements are made by interferometry between the reflected beam and the primary beam. In this case one reflector target point at a time is measured. The system is relatively complicated to set up and relatively complex processing circuitry is required. An older technique for use in surveying railway tracks is the so-called shadow board technique such as described in British patent specification No. 1,322,785. Here again only one point along the track is measured at a time.